This invention relates generally to preionization sources for gas lasers, and, in particular, to a safe radioactive preionization system for gas lasers which produce numerous secondary emission electrons.
Lasers find use in a wide diversity of activities ranging from communication over great distances to the drilling of very accurate holes in objects.
Most lasers consist of a column of active material having a partly reflecting mirror at one end and a fully reflecting mirror at the other. The laser is primed by pumping the atoms of the active material, by means of a flash of intense light, to an excited state. With a preponderance of atoms in that state the system can be stimulated to produce a cascade of photons, all the same wavelength and all in step, by triggering the emission of energy that drops the atoms from the excited state to a lower energy state. A photon carrying this quantum of energy, on striking an excited atom, causes it to emit a photon at the same frequency, and the light wave thus released falls in step with the triggering one. Waves that travel to the sides of the column leave the system, but those that go to the ends of the column along its axis are reflected back and forth by the mirrors. The column, whose length is a whole number of wavelengths at the selected frequency, acts as a cavity resonator, and a beam of monochromatic, coherent light rapidly builds in intensity as one atom after another is stimulated to emit photons with the same energy and direction. After the laser light has built up in this way it emerges through the partly reflecting mirror at one end of the column as an intense highly directional beam.
The active medium of a gas laser, such as, for example, the conventional CO.sub.2 laser is an electrically excited mixture of carbon dioxide, nitrogen and helium. Uniform excitation of the gas mixture at atmospheric pressure, however, is not readily achieved. As the pressure is increased in the conventional low-pressure glow discharge, the characteristics of the discharge change, and at about 200 torr the flow constricts to an arc.
In some instances, a glow discharge can be maintained in the gas by making the discharge time short compared to the arc formation time or by limiting the discharge current density below that required for the formation of a constricted arc. It has been found that for a 1-m discharge length at atmospheric pressure, voltages in the neighborhood of 10.sup.6 volts are required for proper excitation of CO.sub.2 lasers. To meet the requirement of a short discharge time and to lessen the requirement for such extremely high applied voltages, scientists used pulsed transverse excitation, that is, a discharge that is transverse rather than parallel to the optic axis.
Various methods of preionization are used in gas lasers to obtain larger volumes of gas discharge and thus more energy. Preionization refers to the presence of charged particles in the gas volume prior to initiation of the discharge. These charges aid in the initiation of a large volume glow discharge of high spatial uniformity.
In order to avoid arcing and to achieve uniform electrical discharges in gaseous lasers, it is necessary to create a background ionization level well above ambient prior to the initiation of the laser discharge. One method used for achieving this spatially uniform "preionization" is to irradiate the laser gases with ionizing ultra-violet photons. These are commonly generated by weak spark discharges in the vicinity of the lasing volume.
Another method of preionization is one in which the energetic particles emitted by radioactive materials are used to impact ionize the laser gases. Although at first glance this method appears to have substantial merit, there are health safety problems and other drawbacks associated with it. For example, there is the spatial uniformity and ionization efficiency problems associated with the various types of radioactive decay products. Alpha-particles (helium nuclei) do not generate uniform background preionization in gases, but only create very narrow, concentrated ionization paths through the gases. These paths are randomly distributed throughout the gas, and, owing to very rapid electron-ion volume recombination processes in the dense ionization trails, are extremely short lived. Thus, at the instant when the laser discharge is initiated, there are only a few, randomly distributed, highly ionized trails present in the laser gas and a uniform discharge is impossible. Similarly, gammaray photons have very large mean free paths in gases and generate very little volume ionization during their traversals of the lasing volume. As far as beta particles (electrons) are concerned, owing to the exponential drop-off of their ionizing power with increasing beta energy, only the low energy end of the beta particle distribution function contributes appreciably to the preionization process. The high energy betas completely traverse the laser volume without generating sensible amounts of ionization in the laser gases.
In addition, the proximity of radioactive materials to the laser discharge volume subjects such materials to probable bombardment by discharge ions and/or electrons. Such bombardment can dislodge sensible amounts of radioactive material, disperse it throughout the laser discharge volume, and contaminate the laser's component parts. Because of this process, health safety considerations would preclude the use of radioactive materials in gas discharge lasers unless precautions are taken to prevent its occurrance.